The dielectric filter and the dielectric duplexer incorporated in a high frequency circuit are always required to provide miniaturized products.
Various techniques have been provided for achieving the miniaturization. Patent document 1 describes a dielectric filter with an ultra hetero-axial structure in which the axes of a large-diameter hole section and a small-diameter hole section of a stepped resonator hole are largely shifted from each other so as to bend the resonator hole.
By arranging resonators with such resonator holes in a dielectric block, the resonators are coupled together so as to form an attenuation pole. By adjusting the pitch between the resonator holes if necessary, the attenuation pole can be tuned to a desired frequency.
A configuration example of a dielectric duplexer employing this conventional technique is shown in FIG. 1. FIG. 1 is a sectional view of the dielectric duplexer in parallel with the arranging direction of the resonator holes, in which the upper side is an open end face and the lower side is a short-circuit end face.
A dielectric block 1 is provided with a plurality of resonator holes 2A to 2C and 3A to 3C, and an inner conductor is formed on each hole. At ends of the resonator holes 2A to 2C and 3A to 3C, electrode non-forming areas 7 are provided. On the external surface of the dielectric block 1, an external conductor 6 is formed. The inner diameter of each of the resonator holes 2A to 2C and 3A to 3C adjacent to the open end face is large (this portion will be referred to as a large-diameter hole section) while the inner diameter adjacent to the short-circuit end face is small (referred to as a small-diameter hole section below) so as to form stepped holes. In this example, the distance between the resonance holes of the resonator holes 2A to 2C adjacent to the open end face is larger than that adjacent to the short-circuit end face (referred to as a cross-eyed shape below). By such a configuration, two resonators adjacent to each other are inductively coupled together due to the resonator holes 2A to 2C so as to form a transmitting filter. On the other hand, the distance between the resonance holes of the resonator holes 3A to 3C adjacent to the open end face is smaller than that adjacent to the short-circuit end face (referred to as a separate-eyed shape below). By such a configuration, two resonators adjacent to each other are capacitively coupled together due to the resonator holes 3A to 3C so as to form a receiving filter.
The attenuation pole generated by the coupling between the resonators can be adjusted by setting the eccentricity between the small-diameter hole section and the large-diameter hole section and the step ratio, which is the cross-sectional area ratio, between the small-diameter hole section and the large-diameter hole section.
When the dielectric filter and dielectric duplexer, having such an ultra hetero-axial structure, are further miniaturized, the space between the resonators is reduced in accordance with the miniaturizing, so that the wall thickness of the dielectric block is reduced. Accordingly, the capacitance between the resonators is increased. Then, the attenuation pole frequency, which is a filter characteristic, is deviated from a predetermined value, so that predetermined filter characteristics cannot be obtained.
A technique is shown in Patent Document 2 in that by providing an open end-face electrode on the open end face of the dielectric block, the resonators are coupled together. In the dielectric filter having conventional open end-face electrodes, by adjusting the shape of the open end-face electrode so as to regulate the capacitance between the open end-face electrodes, a dielectric block with desirable filter characteristics is achieved.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-256807    Patent Document 2: Japanese Examined Patent Application Publication No. H06-097721
In the ultra hetero-axial structure mentioned above, the eccentricity between the small-diameter hole section and the large-diameter hole section cannot be established to be more than the sum of radii of the small-diameter hole section and the large-diameter hole section. Hence, the range of the obtainable eccentricity is limited. That is, when the dielectric filter and dielectric duplexer, having the conventional ultra hetero-axial structure, are further miniaturized, it has been difficult to achieve required filter characteristics even when the eccentricity is adjusted.
When the eccentricity is cross-eyed, for example, the inductive coupling falls short due to the miniaturizing, so that the desirable bandwidth may not been obtained. When the eccentricity is separate-eyed, the capacitive coupling is excessive due to the miniaturizing and the capacitive coupling is relatively increased, so that the desirable filter characteristics may not been obtained.
Also, the electric current concentration may occur in part of the short-circuit end face, depending on the space between the small-diameter hole sections, deteriorating a Q value.
If open end-face electrodes are provided, when the miniaturization is further executed, the space between the open end-face electrodes adjacent to each other is to be reduced. Accordingly, the capacitance between the open end-face electrodes is increased. Then, in the same way as in the ultra hetero-axial structure, the capacitive coupling is relatively increased, so that it has been difficult to achieve the required filter characteristics. The pattern of the open end-face electrodes is also miniaturized, so that it has also been difficult to form the pattern with high accuracies.
As described above, in the conventional techniques, the design of filter characteristics in accordance with the miniaturization is limited.